1. Field of the Invention
The present invention relates to an asymmetry compensation apparatus for applying asymmetry compensation to a reproduced signal, provided, for example, for a reproduction apparatus for disc-shaped recording media.
2. Description of the Related Art
Conventionally, a so-called asymmetry phenomenon is generated in recording pits ("recording pits" here refer to recording marks formed by magneto-optical recording and phase-change recording as well as physically formed holes as in a compact disc (CD)) formed on a disc-shaped recording medium (hereinafter just called a disc) due to changes in various conditions during recording. This asymmetry adversely affects reproduced data so as to generate an error in the length of each inverted period, and may cause inappropriate data reproduction processing.
Therefore, for compensating for such asymmetry, there has been known an asymmetry compensation circuit shown in FIG. 17.
The asymmetry compensation circuit shown in the figure compensates for asymmetry while it makes an analog reproduced RF signal read from a disc to a binary RF signal (EFM signal in a CD system).
The reproduced RF signal is input to an non-inverted input terminal of a comparator 50 through a coupling capacitor C10. A voltage division point at which a DC power source line voltage is divided by resistors R20 and R21 is also connected to the non-inverted input terminal of the comparator 50.
The comparator 50 compares the level of the reproduced RF signal input to the non-inverted input terminal with a threshold TH input to an inverted input terminal of the comparator to generate and output the binary RF signal. This binary RF signal is sent to a required decoding processing circuit system at the subsequent stage. The binary RF signal also branches to be input to a filter section 51 for asymmetry compensation.
The filter section 51 serves as a low-pass filter (integrator) and provided with a first stage filter formed of a resistor R22 and a capacitor C11 and a second filter formed of a resistor R23 and a capacitor C12. The filter section 51 has the certain time constant determined by the resistances and the capacitances of these elements.
The filter section 51 filters the input binary RF signal and sends it to an amplifier 52. If asymmetry causes an error (dispersion) in the length of the inverted period of the binary RF signal, the filter section 51 outputs the DC component having the level corresponding to this error.
Since the output of the filter section 51 is input to the inverted input terminal of the comparator 50 through the amplifier 52 as the threshold TH, the threshold TH varies according to the effect of the asymmetry. With this threshold TH being input, the comparator 50 performs a converging operation such that the binary RF signal in which the effect of asymmetry is canceled is output.
In recent years, a disc called a digital versatile disc ROM or a digital video disc ROM (DVD-ROM) has been developed as an optical-disc recording medium suited to multimedia use. There also have been proposed a rewritable recording medium which is compatible with this DVD-ROM and which does not make the configuration of a recording and reproduction apparatus complicated, and a recording and reproduction system therefor.
In such a recording and reproduction system for a rewritable medium, the minimum recording unit of data is specified in a format. For example, as shown in FIG. 18A, data is written in units of blocks, which serve as the minimum units of recording data. In the figure, data is continuous in the order of Block (N-1), Block (N), and Block (N+1). To avoid overlap between the last data of the preceding block and the first data of the current block, a link area having the specified size, called a linking section, is provided.
According to such a format, a recording sequence from the start of data recording to the end is completed in units of blocks at the minimum length.
Assume that data is recorded not continuously but by different recording sequences in Block (N-1), Block (N), and Block (N+1) in FIG. 18A.
In this case, since recording is performed for Block (N-1), Block (N), and Block (N-1) at different chances, each block may be recorded by a different driver. Even if the same driver is used, an error in laser power in recording or an environment change such as a change in the ambient temperature can be considered.
When data recorded on the same disc with different recording conditions such as described above is reproduced, a characteristic difference such as a change in the amplitude level of a reproduced RF signal is generated between blocks. FIG. 18B roughly shows a reproduced RF signal obtained by reproducing the data shown in FIG. 18A. The width W in the up and down directions of the signal indicates its amplitude level. It is understood from the figure that the amplitude level of a reproduced RF signal may change between blocks if Block (N-1), Block (N), and Block (N+1) are recorded by different recording sequences.
When the asymmetry compensation circuit having the configuration shown in FIG. 17 is applied to a system in which data is substantially sequentially recorded in a medium, such as a CD system, there occurs no problem if the time constant of the filter section 51 is fixed. On the other hand, if the asymmetry compensation circuit having the configuration shown in FIG. 17 is applied to a case in which the reproduction state shown in FIG. 18B is generated, although the ideal threshold TH should be as shown by a solid line in FIG. 18B, since the filter section 51 has a fixed time constant, the threshold level actually does not respond such swiftly and changes to the threshold level TH appropriate for Block (N) with some time difference as shown by a dotted line "a" when reproduction proceeds from Block (N-1) to Block (N). In this case, a binary RF signal for which appropriate asymmetry compensation is not performed is output during the period indicated by the dotted line "a." This means that reliability of read data deteriorates accordingly.